In any optical network it is important to maintain correct power levels for all traffic channels. This is generally achieved by monitoring the output power of optical amplifiers using a broadband optical detector, such as a photodetector. The monitored output power is then utilised in a feedback loop to adjust the amplifier gain so that the desired output power is produced. This principle is illustrated in FIG. 1. The arrangement in FIG. 1 includes an optical amplifier 10, which in the illustrated example is an active fibre amplifier that is driven in the conventional manner by one or several pump lasers 20. The amplifier 10 may equally be a semiconductor laser amplifier in which case the pump laser 20 would be replaced by a current or voltage supply controlling the laser. The amplifier 10 receives an input signal power Pin. The output of the amplifier 10 is split using an optical coupler or beam splitter 30 or other conventional means to extract a small proportion of the output signal. This is fed to an output monitor 40, such as a broadband photodetector, which converts the optical signal into an electrical signal. This signal having power Pout is then compared with a desired, or target, signal power Pout—target. This target power Pout—target is the total power of all channels transmitted in the optical fibre. It can thus be expressed asPout—target=Nch×Pch—target where Nch is the number of channels carried in the WDM link.
However, a characteristic of optical amplifiers, whether they comprise fibre amplifiers or semiconductor lasers, is amplified spontaneous emission (ASE) which manifests itself as a broadband signal at the amplifier output. For high input signal powers, the power measured at the output includes a negligible proportion of amplified spontaneous emission (ASE). However, at low signal powers, for example powers lower than about −20 dBm, the proportion of the total measured output power due to ASE is important. If the signal output power is corrected by adjusting the amplifier gain on the basis of this target power Pout—target as described above with reference to FIG. 1, the resultant channel output power will inevitably be lower than required.
It is known to utilise a narrow band detector at the output of the amplifier to measure the signal power at a limited range of wavelengths. This effectively filters the ASE out so that the monitored signal is a faithful copy of the output traffic power. While such a solution is possible in systems using a single carrier wavelength, such as time domain multiplexed (TDM) systems, this is not the case for WDM systems where a large number of different wavelengths are used. While narrow band detection may be employed for one of the signal wavelengths present in the WDM system, this is problematic for two reasons. Firstly, the system becomes inflexible, since the monitored signal must be routed through all the optical amplifiers in the network. Secondly, the system is inherently frail because any fault occurring in the monitored channel will result in the collapse of the whole network.
There is thus a need for a means of stabilising the output power of optical amplifiers that is effective in WDM systems, simple to implement and largely insensitive to faults in individual channels.